It is desirable in injection molding to urge the molten thermoplastic resin material into contact with the mold surfaces by exerting pressure on the resin. This aids the external surface of the plastic material in assuming the precise shape dictated by the mold surface. The pressure also assists the filling of the mold with molten resin even if the mold cavity is elongated or narrow and/or is otherwise difficult to fill.
In gas assisted injection molding, the articles are produced by injecting molten resin into the mold cavity and then injecting a quantity of pressurized gas into the resin to fill out the mold cavity and form a hollow gas space in the resin. The gas is preferably an inert gas such, for example, as nitrogen. Pressure is maintained on the gas in the hollow gas space within the resin until the resin has sufficiently set whereafter the pressurized gas is released from the molded part hollow space and the molded part is removed from the mold cavity.
The gas assisted procedure is advantageous since the molded part produced utilizes somewhat less plastic material and is lighter than if the part were solid plastic. More importantly, the plastic in the gas assisted procedure will not have a tendency to shrink away from the mold walls during cooling since the internal gas pressure will keep it pressed against the walls, thereby minimizing or eliminating surface blemishes such as sink marks. Further, the gas assisted procedure eliminates the need to utilize the screw ram of the injection molding machine to pack out the mold during the molding cycle, thereby minimizing or eliminating molded-in stresses in the molded part.
Whereas the gas assisted injection molding process offers many advantages, some of which are enumerated above, as compared to injection molding without gas assistance, the known gas assistance processes embody certain limitations and disadvantages.
Specifically, in the gas process it is desirable to inject a gas from a location upstream of the mold cavity to fill out the mold cavity along the general flow path of the injected resin, and preferably, to inject the gas at the resin injection location so as to use a portion of the resin contained in the sprue and runner for material savings. However, venting of the gas through the gas injection passageways may entrain resin particles with the result that the runner, sprue, or injection nozzle passageways may become blocked. Blockage of these passageways results in down time of the apparatus to remove the blockage and further results in incomplete venting of the molded article with the result that the molded article may rupture, due to internal pressure, when the mold is opened.
Blockage of the runner, sprue and nozzle passageways may be minimized or eliminated by delaying the venting operation until such time as the resin has generally hardened, but this delay lengthens the cycle time for each part and increases the cost of each part. It has been proposed to inject the gas directly into the mold cavity and vent the gas directly from the mold cavity. Whereas this arrangement may allow earlier venting with resultant savings in per part cost, it does not permit usage of resin contained within the sprue and runner to fill out the mold cavity, nor does the gas readily flow through the resin upstream of the injection location so that the resin is not adequately packed out against the cavity wall at locations upstream of the injection location and may pull away from the cavity wall, with resultant sink marks, during the cooling process.